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Digital links display only a band of spectral occupancy visually similar to filtered white noise. It is not immediately evident from RF spectra whether any of the separate base-band channels are occupied or not. It is necessary to discuss the construction of these signals before any analysis is considered.
The source will typically be a single multiplexed digital signal as described by ITU-T Recommendation G.703. This describes the data and framing protocol of land line digital links. Since the transmitter has no control over the nature of the digital signal, it is exclusively ordered by a pseudo-random digital sequence generated within the system. The reason for this is to avoid situations of a no data microwave signal (spectral dispersion) and, in some cases, to provide sufficient digital transitions for clock recovery. There is no global specification for this randomizing sequence that differs with equipment type and manufacturer.
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